Contents

Mechanisms

Transpirational pull results ultimately from the evaporation of water from the surfaces of cells in the interior of the leaves. This evaporation causes the surface of the water to recess into the pores of the cell wall. Inside the pores, the water forms a concave meniscus. The high surface tension of water pulls the concavity outwards, generating enough force to lift water as high as a hundred meters from ground level to a tree's highest branches. Transpirational pull only works because the vessels transporting the water are very small in diameter, otherwise cavitation would break the water column. And as water evaporates from leaves, more is drawn up through the plant to replace it. When the water pressure within the xylem reaches extreme levels due to low water input from the roots (if, for example, the soil is dry), then the gases come out of solution and form a bubble - an embolism forms, which will spread quickly to other adjacent cells, unless bordered pits are present (these have a plug-like structure called a torus, that seals off the opening between adjacent cells and stops the embolism from spreading).

Measurement

Until recently, the negative pressure (suction) of transpirational pull could only be measured indirectly, by applying external pressure with a Scholander bomb to counteract it[1]. When the technology to perform direct measurements with a pressure probe was developed, there was initially some controversy about whether the classic theory was correct, because some workers were unable to demonstrate negative pressures. More recent measurements do tend to validate the classic theory, for the most part. Xylem transport is driven by a combination of transpirational pull from above and root pressure from below, which makes the interpretation of measurements more complicated.

A common misconception is that water moves in xylem by capillary action—the movement of water along a small-diameter conduit (such as a capillary) as a result of surface tension in the meniscus at the leading surface of the moving water. Surface tension does play a critical role in water movement in xylem, as described above, but the relevant force acts at the surface site of evaporation within leaves, not within the xylem conduits. Water movement within the xylem conduits is driven by a pressure gradient created by such force, not by capillary action. Specifically, the evaporation and transpiration of water in the leaves causes water in the xylem to move from the roots, which have a higher water potential, up the stem of the plant that has a decreasing water potential along its length.

Water is constantly lost by transpiration in the leaf. When one water molecule is lost another is pulled along by the processes of cohesion and adhesion. Transpiration pull, utilizing capillary action and the inherent surface tension of water, is the primary mechanism of water movement in plants. However, it is not the only mechanism involved. Any use of water in leaves produces forces water to move into them.

N. Michele Holbrook; Michael J. Burns, and Christopher B. Field (November 1995). "Negative Xylem Pressures in Plants: A Test of the Balancing Pressure Technique". Science270 (5239): 1193–4. doi:10.1126/science.270.5239.1193. is the first published independent test showing the Scholander bomb actually does measure the tension in the xylem.

Pockman, W.T.; J.S. Sperry and J.W. O'Leary (December 1995). "Sustained and significant negative water pressure in xylem". Nature378: 715–6. doi:10.1038/378715a0. is the second published independent test showing the Scholander bomb actually does measure the tension in the xylem.